Conventionally, there exists a molded motor, having a housing mold formed on a stator using a molding resin, in which a stator core is wound with a coil, and inside of which a rotor is held by a bearing.
Regarding the aforementioned molded motor, because the aforementioned molded motor has its outside surfaces covered with a molding resin that is an insulator, and also, because it is typically employed at lower voltages, there has conventionally not been much need for considering a grounding issue.
However, such a need arises in the case of a molded motor for operation at high voltages and PWM (PULSE WIDTH MODULATION) control. Due to the distribution of capacitance elements resulting from the structure of the motor, as well as the voltage variance (dv/dt) from the PWM, electricity charges the stator, and this electricity flows to the rotor from the stator, via the bearings. Thus, the resulting electro-erosion deteriorates or possibly breaks down the bearings.
For instance, as shown in FIG. 2, in the molded motor 100 for high voltages and PWM (PULSE WIDTH MODULATION) control, wherein an insulating, unsaturated polyester is used as the molding resin, electric current generated due to the aforementioned reason (shown by the dotted arrows in FIG. 2), goes through the stator core 102, the stator coil 104, the bearings 106, the rotary axis 108, and the rotor 110, and returns to the stator core 102. In this manner, electricity passes through the bearings 106, and the resultant electro-erosion deteriorates them or possibly breaks them down.
In consideration of the above-mentioned problem, the present invention offers a brushless direct current (DC) motor where the bearings do not experience electro-erosion. The present invention provides such a motor, covered by a molding resin, where an electric current does not flow from the stator to the bearings 106.